Carbon disulfide (CS2) conversion testing is the definitive performance evaluation for organic sulfur hydrogenation conversion catalysts used in syngas, coke-oven gas, and natural gas purification systems. This article explains the underlying reaction chemistry, sample categories, testing items, fixed-bed evaluation procedures, analytical standards, and industrial applications of CS2 conversion detection, providing engineers, quality professionals, and catalyst developers with a practical technical reference.
Technical Overview
Organic sulfur hydrogenation conversion catalysts—most commonly cobalt–molybdenum (Co-Mo) or nickel–molybdenum (Ni-Mo) oxide formulations supported on high-surface-area alumina—are installed upstream of zinc oxide desulfurization beds in gas purification trains. Their function is to convert refractory organic sulfur compounds, including carbon disulfide (CS2), carbonyl sulfide (COS), mercaptans, sulfides, and thiophenes, into hydrogen sulfide (H2S), which is readily captured by downstream sorbents.
In the activated (sulfided) state, the active phases exist as MoS2 layers promoted by Co or Ni species. Under typical operating conditions of 200–400 °C in a hydrogen-containing atmosphere, the principal hydrogenation reactions include CS2 + 4 H2 → CH4 + 2 H2S, while hydrolysis-type catalysts operating at lower temperatures promote CS2 + 2 H2O → CO2 + 2 H2S.
Among the organic sulfur species encountered in coal-derived and refinery gases, carbon disulfide is widely recognized as one of the most difficult to convert due to its stable molecular structure and strong carbon–sulfur bonds. Consequently, the CS2 conversion rate measured under standardized laboratory conditions is adopted throughout the industry as the most demanding and representative activity index for this catalyst class. A catalyst that achieves high CS2 conversion will almost invariably perform well against less refractory species such as COS and mercaptans.
Testing Samples
Fresh Catalyst (As-Manufactured)
Samples taken from production lots in the oxide form for factory quality control, lot release, and acceptance testing. These samples establish the baseline activity guaranteed by the manufacturer before sulfidation and service exposure.
Pre-Sulfided Catalyst
Catalysts sulfided off-site or activated in the laboratory to the active MoS2 phase before activity measurement. Pre-sulfided samples allow direct evaluation of intrinsic activity and shorten reactor startup studies.
Used (In-Service) Catalyst
Samples withdrawn from industrial reactors after extended operation to assess residual CS2 conversion activity, identify deactivation mechanisms such as coking, sulfation, sintering, or poisoning by chlorides, ammonia, or heavy metal contaminants, and to estimate remaining service life.
Laboratory and Research Samples
Experimental formulations, accelerated-aging specimens, and benchmarking samples evaluated during catalyst development, competitive screening, and optimization of promotional additives or support modifications.
Testing Items
The evaluation program for organic sulfur hydrogenation conversion catalysts combines activity testing with comprehensive physicochemical characterization:
- CS2 conversion rate at specified temperature, pressure, space velocity, and inlet concentration (primary activity index)
- COS hydrolysis and hydrogenation conversion
- Thiophene, mercaptan, and total organic sulfur conversion
- BET specific surface area, pore volume, and pore size distribution
- Side crush strength and particle integrity before and after thermal or hydrothermal treatment
- Bulk (packed) density and particle size distribution
- Chemical composition, including MoO3, CoO/NiO, promoter oxides, and support content
- Total sulfur content and degree of sulfidation
- Attrition loss and abrasion resistance
- Thermal and hydrothermal stability, including activity retention after accelerated aging
Testing Methods
CS2 conversion evaluation is performed in laboratory fixed-bed reactor systems that simulate industrial service conditions. A typical test protocol comprises the following stages:
- Sample preparation — catalyst is sampled representatively, optionally crushed and sieved to a defined fraction (commonly 0.4–0.8 mm) or tested as whole extrudates, then dried at 105–120 °C to remove moisture
- Sulfidation (activation) — the oxide catalyst is heated in hydrogen or H2/N2 blended with a sulfiding agent such as CS2 or dimethyl disulfide, ramped to approximately 300–350 °C and held until breakthrough indicates complete conversion to the sulfided active phase
- Activity measurement — a feed gas of N2 or H2/N2 containing a defined CS2 concentration (commonly 50–1000 mg/Nm³) is passed over the catalyst at 200–400 °C, pressures from atmospheric to about 3.0 MPa, and gas hourly space velocities typically between 500 and 5000 h⁻¹, with conditions selected according to the applicable specification
- Product analysis — inlet and outlet CS2 concentrations are quantified by gas chromatography with flame photometric detection (GC-FPD), pulsed flame photometric detection (GC-PFPD), sulfur chemiluminescence detection (GC-SCD), or microcoulometric titration
- Calculation — CS2 conversion is calculated as X = (Cin − Cout) / Cin × 100%, supported by carbon and sulfur mass-balance verification to confirm measurement integrity
Relevant analytical references include ISO 19739, ASTM D5504, and ASTM D6228 for the determination of sulfur compounds in natural gas and gaseous fuels by gas chromatography, together with applicable national and industry standard methods for gas purification catalysts. Measurement reliability is ensured through calibration with certified CS2 standard gases, blank and duplicate runs, and cross-verification by accredited third-party laboratories.
Applications
CS2 conversion testing supports catalyst qualification and process reliability across a broad range of industries:
- Coal chemical industry — purification of coal gasification gas and coke-oven gas ahead of shift conversion, methanation, and methanol synthesis
- Synthetic ammonia plants — protection of low-temperature shift and methanation catalysts from sulfur poisoning
- Hydrogen production — feed pretreatment for steam reformers and pre-reformer protection
- Natural gas processing — removal of trace organic sulfur upstream of sweetening, liquefaction, and distribution
- Petrochemical hydrotreating — feed polishing and guard-bed catalyst evaluation
- Catalyst manufacturing and research — product qualification, lot release, competitive benchmarking, and service-life studies
FAQ
Why is carbon disulfide used as the probe molecule for activity testing?
CS2 is among the most thermodynamically stable and hydrogenation-resistant organic sulfur compounds present in industrial gases. Because it is harder to convert than COS, mercaptans, or thiophenes, a high CS2 conversion value demonstrates catalyst capability against the full spectrum of organic sulfur species, making it the industry-preferred benchmark.
What conversion level is typically required?
Commercial specifications commonly require CS2 conversion of at least 99%, although thresholds such as 95% or 90% may apply depending on service severity and feed sulfur concentration. The acceptance criterion must always be interpreted together with the corresponding test conditions of temperature, pressure, space velocity, and inlet concentration.
Must the catalyst be sulfided before testing?
Yes. Co-Mo and Ni-Mo catalysts are catalytically active only in the sulfided form; the as-delivered oxide must first be converted with a sulfiding agent such as CS2 or dimethyl disulfide in hydrogen. Test protocols therefore include a controlled sulfidation stage before activity measurement, unless the product is supplied pre-sulfided.
How much sample is required and what is the typical turnaround?
A representative sample of approximately 50–200 g, obtained according to a defined sampling plan, is generally sufficient for a complete evaluation. A standard test including sulfidation, steady-state activity measurement, and chromatographic analysis typically requires several working days, while extended aging or life-cycle studies take longer.
What is the difference between hydrolysis and hydrogenation conversion catalysts?
Hydrolysis catalysts, often titanium dioxide or alumina based, convert COS and CS2 with steam to H2S and CO2 at lower temperatures, whereas sulfided Co-Mo/Ni-Mo hydrogenation catalysts require hydrogen and convert the complete range of organic sulfur compounds, including thiophenes that resist hydrolysis. Test gas composition and evaluation conditions differ accordingly.
Can used catalyst samples be evaluated?
Yes. Used catalyst testing combines residual CS2 conversion activity measurement with physicochemical characterization to identify deactivation causes—such as coke deposition, sulfate accumulation, active-phase sintering, or contaminant poisoning—and to support decisions on regeneration or replacement.